1. Field of the Invention
The present invention relates to a method for bonding a wire and a bonding apparatus, more particularly, to the method for bonding a metal wire onto a material (to be bonded), such as an aluminum electrode of a semiconductor chip, by means of a pressing force and heat energy.
2. Description of the Related Art
The method for bonding a wire described above is generally performed by the processes shown in FIGS. 4A to 4E.
After a copper ball 4 is formed on the end of the copper wire 2, which is 25 to 30 .mu.m in diameter, which passes through a capillary tip 1, by the discharge energy of a torch electrode 3 (FIG. 4A), the capillary tip 1 is lowered to press the copper ball 4 against an aluminum electrode 8 of a semiconductor chip 7 attached to a die pad 6 of a copper lead frame 5 and is deformed plastically (FIG. 4B). During this time, the semiconductor chip 7 is heated to a temperature of 300.degree. C. to 400.degree. C. by a heat block 9 which the die pad 6 is placed on, and ultrasonic vibration is applied to the capillary tip 1 by a vibrating device (not shown), to diffuse metallic elements of the copper ball 4 and the aluminum electrode 8 mutually, and produce a bond between the copper ball 4 of the copper wire and the aluminum electrode 8. FIG. 5 shows the ball bonding area in large scale.
Next, the capillary tip 1 is raised, which causes the copper wire 2 to feed through the capillary tip 1 (FIG. 4C), and the tip is moved over and down to the surface 11 of the inner lead 10, and the copper wire 2 is then pressed against and bonded to the inner lead 10 (FIG. 4D). This is called the stitch bond and looping method. During this time, the inner lead 10 is heated to a temperature of 300.degree. C. to 400.degree. C. by the heat block 9 supporting the inner lead 10, and ultrasonic vibration is applied to the capillary tip 1 by the vibrating device (not shown), to mutually diffuse metallic elements of the copper wire 2 and the surface 11 of the inner lead 10 to produce a bond between the copper wire 2 and the surface 11 of the inner lead 10. FIG. 6 shows the stitch bonding area in large scale.
Thereafter, the clamper 12 is closed to clamp the wire 2. Next, the capillary tip 1 and clamper 12 are raised, whereby the copper wire 2 is pulled by the clamper 12 to sever the copper wire 2 (FIG. 4E).
Thus, the known wire bonding method described above is an ultrathermosonic bonding method. This method applies the pressing force, heat energy and ultrasonic vibration by the capillary tip 1 to provide for the mutual diffusion of the metal wire and the material (to be bonded). They are bonded with each other through the alloy layer. Ultrasonic vibration will work on the two metallic elements, the metal wire and the material (to be bonded), to diffuse these elements fast. Comparison between the above described ultrasonic bonding method and the thermocompression wire bonding method: This ultrathermosonic bonding method is superior to another wire bonding method known as thermocompression method from the time viewpoint. Specifically, since the thermocompression bonding method applies
only a pressing force and heat energy, but for mass production of semiconductor devices, it is difficult to use.
Two different bonding samples were prepared; one was made by the ultrathermosonic bonding method and another was made by the thermocompression bonding method, and they were molded in an epoxy resin. While they had been stored at a high temperature from 150.degree. C. to 250.degree. C., the electric resistance of the bonding area had been measured. The results of these experiments indicated that the former bonding device resistance increased faster than the latter one. This has also been described in a report entitled "Reliability Evaluation of the bonding Au Wire and Al Electrode" in Technical Report NS-20568 issued in January in 1985 by Nippon Tsushin Gijutsu K.K.
It is not easy to deform the material plastically (to be bonded) by ultrathermosonic bonding method. For example, when the material (to be bonded) is an aluminum thin film electrode 8 such as shown in FIG. 5, the aluminum electrode 8 may be depressed under the metal ball and the contact area between the ball 4 and the aluminum electrode 8 may become smaller. This may result in bad electric conductivity, because a little corrosion of this bonding area will terribly reduce the bonding reliability and finally break this contact.
On the other hand, in the thermocompression bonding method; a pressing force and heat energy without ultrasonic vibration is applied. In this method, the higher the heat energy used, the shorter the bonding time required. But, with the heat block 9 heated to a higher temperature, there is a possibility that the bonding material between the die pad 6 and the semiconductor chip 7 may become softened or decomposed, and that the semiconductor chip 7 may become functionally damaged. Accordingly, it has been difficult to reduce the bonding time required.